Field of the Invention
This invention relates to a Waelz process of volatilizing zinc and lead from iron-containing and oxide-containing materials in an inclined rotary kiln, which is charged at its upper end with the material together with solid carbonaceous reducing agent having a high content of volatile combustible constituents, wherein the reduction and volatilization are effected below the softening point of the charge, metallic iron-containing material is discharged from the lower end of the rotary kiln, combustion air is sucked into the lower end of the rotary kiln, and the exhaust gases are sucked from the upper end. More especially, this invention relates to such a process wherein oxygen-containing gas is introduced directly into the charge through air nozzles over which the charge is disposed at a point defined by the point where ignitable particles of the reducing agent first appear and 50 percent of the length of the kiln.
The Waelz process is used for a treatment of raw materials which contain volatilizable metals or volatilizable metal compounds, mainly for a reduction and volatilization of zinc and lead from relatively low-metal precursors, such as corresponding ores, intermediate products, slags from lead blast furnaces, retort residues, etc. The reaction unit consists of an inclined rotary kiln, which is operated continuously. The reducing agent typically consists of solid carbon, which is fed with the charge, and optional carbon contained in the raw materials. The required combustion air is sucked into the rotary kiln from its discharge end. The exhaust gases leave the furnace through the charging end. Two interdependent reactions are performed in the charge: the reduction of, e.g., zinc oxide by means of carbon monoxide EQU ZnO+CO=Zn+CO.sub.2
and the Boudouard reaction of the resulting carbon dioxide EQU CO.sub.2 +C=CO
to form new CO for the reduction. Part of the resulting CO and the zinc, which becomes available at a temperature above its boiling point, enter the free kiln space above the charge and are burnt there. The resulting zinc oxide dust is collected from the exhaust gas.
Iron oxides contained in the raw materials are reduced too. Most of the iron is contained as metallic iron in the material discharged from the kiln. Whereas in the Renn Waelz process, the charge has a pasty consistency in the final zone of the rotary kiln and the metallic iron agglomerates to form balls or spheres, the metallic iron is finely divided in the material discharged from the kiln in the Waelz process and if said material has a sufficiently high iron content, it can be used to produce crude iron and steel.
The Waelz process has been used more recently on an increasing scale in the recovery and re-use of the residual substances which inevitably become available in the production of crude iron and steel and may consist of dusts and muds obtained from blast furnace plants, sintering plants, steel making and rolling mill plants. That practice eliminates the need for dumping the substances and permits re-use of their metal contents in a high degree. The solid material discharged from the rotary kiln contains metallic iron and is generally charged to a blast furnace.
It has been believed for a long time that only coke breeze and anthracite can be used as reducing agents in the Waelz process. West German Offenlegungsschrift 26 21 969 describes a process in which a mixture of a carbonaceous material which reacts quickly and a carbonaceous material which reacts slowly is fed as solid reducing agent into the upper end of the rotary kiln. The proportion of the carbonaceous material which reacts slowly is about 20 to 80 percent, particularly 40 to 60 percent, based on fixed carbon. Reducing agents which react quickly are, e.g., brown coals or lignites. Carbonaceous reducing agents which react slowly are, e.g., anthracites, long-flaming gas coals, and coke. The maximum temperature in the kiln can be adjusted between 900.degree. and 1050.degree. C., and preferably between 950.degree. and 1000.degree. C. The combined use of reducing agents which react quickly and slowly, respectively, permits of a higher throughput rate and decreases the incrustation, results in a relatively low carbon surplus of carbon in the material discharged from the rotary kiln, and prevents a sintering of the charge and a reoxidation of the metallic iron which has been formed. When more heat from the kiln gases is offered in the reduction zone, the temperature of the charge will not increase in that zone until substantially all oxygen has been removed from the oxides. In that zone, virtually all heat which is offered by the kiln gases and the kiln wall is consumed in the highly endothermic Boudouard reaction within the charge so that no heat is available for a temperature rise of the charge. In that region the temperature curve of the charge is virtually horizontal in the longitudinal direction of the kiln and rises only toward the discharge end, when almost all oxygen has been removed from the oxides. Because the reducing agents have different reactivities, the temperature level is decreased as the reducing agents which react quickly tend to decrease the temperature whereas the remaining reducing agents which react slowly prevent a carbon deficiency in the final zone.
In a modified Waelz process, the rotary kiln is provided with a continuously operated central burner and with shell tubes for continuously supplying air into the kiln axis and anthracite is used as a reducing agent ("Erzmetall", Vol. 23 (1970) No. 3, pages 108-113).
It is an object of the invention to provide a Waelz process which results in a higher throughput rate and permits of the use of inexpensive reducing agents.